1. Technical Field
The present disclosure relates to a terminal, a base station, a transmission method, and a reception method.
2. Description of the Related Art
In an LTE-Advanced technique, which is further enhanced from a 3rd Generation Partnership Project Radio Access Network Long Term Evolution (3GPP-LTE; hereinafter, referred to as LTE), accommodating increased traffics as described below is considered. That is to say, in an area covered by macro cells that are base stations with high transmission powers (referred to as eNBs), small cells that are base stations with low transmission powers (referred to as low power nodes (LPN) in some cases) are arranged in high density.
For operating the high-density arrangement of small cells, control of the on state and the off state of the small cells is considered in order to suppress interference caused by the small cells and to reduce the power consumed by the small cells. When the small cells are in the off state, the small cells are in a “halt state” in which data is not allocated to a terminal (user equipment (UE)).
However, when transmission of all signals from a small cell is stopped, the terminal cannot detect the small cell any more. To prevent this, causing the small cell to transmit a cell detection reference signal (referred to as discovery signal in some cases) is considered so that the terminal can detect a small cell in the off state. The small cell transmits the cell detection reference signal to the terminal to cause a network to report a measurement result in the terminal. With this, a base station can appropriately set a cell connection of the terminal taking into account propagation path conditions between the terminal and the small cells or traffic conditions of each small cell.
The cell detection reference signal is a signal for performing cell detection in the terminal, time frequency synchronization, and measurements for radio resource management (RRM) of same frequencies and different frequencies when the small cell is in the off state. To suppress interference with the terminal and the power consumed by the small cell, the cell detection reference signal is desired to be transmitted from the small cell with long transmission intervals. Furthermore, for the measurements for RRM to be performed in the terminal, causing the transmission interval of the cell detection reference signal to be notified in advance from the network is considered.
The small cell in the off state transmits only the cell detection reference signal and does not transmit any other signals. Therefore, an existing terminal (legacy UE supporting the standard specification for Rel. 11 or before, for example) cannot use the cell detection reference signal for the measurements for RRM. By contrast, a new terminal (UE supporting the standard specification for Rel. 12, for example) responds to the measurements for RRM using the cell detection reference signal for the purpose of enhancement of small cells, and thus can observe the cell detection reference signal. Furthermore, the small cell in the on state transmits the cell detection reference signal together with synchronization signals (primary synchronization signal (PSS)/secondary synchronization signal (SSS)) or cell specific reference signal (CRS). With this, the existing terminal and the new terminal both can observe a signal.
Furthermore, as a configuration of the cell detection reference signal, changing the cycle, the band (that is, time and frequency resources), and the like of a signal already present in an LTE-Advanced system for use is considered. As a candidate signal used as a cell detection reference signal, a positioning reference signal (PRS), a channel state information-reference signal (CSI-RS), a reduced PSS/SSS+CRS, and the like are cited (see 3GPP TSG RAN WG1 meeting, R1-133457, NTT DOCOMO, “Small Cell Discovery for Efficient Small Cell On/Off Operation”, August 2013, for example).
As an example, a case where the CSI-RS is applied for the cell detection reference signal will be herein described.
The CSI-RS is a signal introduced for the purpose described below. A terminal uses the CSI-RS to observe channel information from a base station to the terminal and generates a feedback signal in accordance with the observed value to report to the base station. The base station performs adaptive modulation, precoding control, or the like based on the feedback signal. The CSI-RS is defined in a configuration corresponding to a maximum 8-port transmission antenna of the base station (see 3GPP TS36.211v11.4.0 (2013-09), “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation (Release 11)”, September 2013, for example). More specifically, the CSI-RS is defined in each of configurations for 8 ports, 4 ports, and 2 ports in accordance with the number of the transmission antenna ports of the base station.